rbg ram concept guide

RBG Guide to Using RAM Concept

CONTENTS CONCRETE CODES AND SOFTWARE VERSION..................................................1

PURPOSE OF THIS GUIDE ........................................................................................2

MESH INPUT................................................................................................................2

Slabs...............................................................................................................................2

Local slab thickenings at columns .................................................................................2

Columns .........................................................................................................................3

Walls ..............................................................................................................................3

Walls above which begin at the slab..............................................................................4

Concrete Balustrades (and similar thin upstands)..........................................................4

Slab Folds.......................................................................................................................4

Pour Strips, Construction and Movement Joints............................................................4

Temporary penetrations (such as for cranes and hoists)................................................5

LOAD COMBINATIONS.............................................................................................5

LOADINGS ...................................................................................................................5

PATTERN LOADING ..................................................................................................5

CREEP FACTOR ..........................................................................................................6

DESIGN STRIPS...........................................................................................................7

REINFORCEMENT......................................................................................................9

POST TENSIONING...................................................................................................10

SERVICE DESIGN .....................................................................................................11

Viewing Deflections ....................................................................................................11

Long-Term Deflections ................................................................................................11

Cracking and Crack Widths.........................................................................................12

ULTIMATE DESIGN .................................................................................................13

TROUBLESHOOTING...............................................................................................13

Guide to using RAM Concept Page 1 of 14 (JN & JAL - 2010)

CONCRETE CODES AND SOFTWARE VERSION

Software Version considered in this guide

Version 3.1

Note: the latest version of RAM is actually Version 3.3 which includes AS3600-2001

Amendment 2 as well as the new Load History Calculations feature.

Unless noted otherwise, all references in this guide to the RAM Concept User Manual

or Help are to those from version 3.1

Concrete Codes used by RAM

Australian Code

RAM Concept v3.1 does design to AS3600-2001+A1 only

Creep Factors are to be calculated using AS3600-2009

Unless noted otherwise, all references in this guide to the Australian Code are to

AS3600-2009

British Code

BS8110-1:1997

BS8110-2:1985

TR43 1st Edition

Unless noted otherwise, all references to British Codes in this guide are to those

listed above except for TR43 where all references are to the 2nd

edition.

American Code

RAM also does ACI-318 design but this has not been discussed in this guide.


PURPOSE OF THIS GUIDE

This guide is intended to establish a consistent practice for the use of RAM Concept

across Robert Bird Group. This guide is not a company design guide and it does not

replace the design codes. Further to this, it does not replace the RAM Concept User

Manual published by Bentley Engineering, which can be found under the “Help”

menu in RAM. It is advisable to refer frequently to the official manual, especially

when using an aspect of the program or modelling a certain situation for the first time.

MESH INPUT

The most efficient way to create the finite element model is to draw objects on the

Mesh Input plan which represent the various concrete elements you wish to model,

and then to click “Generate Mesh”.

Slabs

Slabs should be modelled to the centrelines of supports. In the mesh, walls are line

supports (point supports in the case of columns) which provide support at the nodes of

the slab finite elements. The “Width” property of a wall object is used to determine

the stiffness of the wall, but in terms of the wall location in plan, the width is only

shown for visual effect; so if there is slab on only one side of a wall and it is modelled

to the back face of that wall, there will actually be a small portion of slab which will

cantilever beyond the line of support. The default stiffness setting is for slabs to be

isotropic, however the stiffness in each direction can be adjusted by selecting

“Custom” under the “Behaviour” tab and adjusting the stiffnesses (the lowest value

possible is 0.001).

Beams

When modelling beams it is usually best to use a beam object rather than a slab. This

is because RAM gives each slab element a torsional stiffness that is proportional to its

depth cubed, however for a beam the actual torsional stiffness is proportional to the

cube of the lesser of the depth or width. If a beam that is deeper than it is wide is

modelled with slab elements instead of beam elements, then it may have an

unrealistically high torsional stiffness resulting in unconservative deflections in

adjacent slabs.

Local slab thickenings at columns

Slab thickenings, regardless of their size on plan, will affect the relative stiffness of

the structure and therefore the moment distribution, often resulting in an increased

negative moment at the column. This is not an error and the slab should be designed

accordingly.

Shear errors can often arise because such local thickenings usually mean there are

different slab thicknesses across the width of the design strip. This will result in a

reduction of the shear area used in the calculations. This can usually be avoided by

setting design strip “section trimming” property to be “slab rectangle” or “max

rectangle.”


Note that for slab thickenings the design strip “inter-slope angle” property should

generally not be set to zero (default value is 0.25) otherwise the full depth of the drop

panel at the column face will not be used for the calculations. If this is done

incorrectly then overly-high amounts of serviceability reo and even service failures

may be called up.

Columns

Generally speaking, only the columns below should be included when first creating a

model. These should be modelled as pin supports by leaving “Fixed Near” unticked in

the column object properties.

“Compressible” should be ticked for better results, and “Roller at Far End” should not

generally be ticked, especially for PT design as this will ignore the effect of columns

taking P/A out of the slab.

The size/height/concrete grade of columns below the slab only have an effect if the

column is modelled to have stiffness (“Fixed Near” must be ticked, and a column

stiffness factor > 0 entered). If a column is modelled with stiffness, then it will take

moment from the slab, which must be taken into account both in the slab punching

shear calculations and in the column design.

Columns above can take moment out of the slab if modelled with fixity, however if

the intention is to reduce slab deflections then the method outlined in SERVICE

DESIGN – Long Term Deflections should be used.

Walls

Adjoining walls should be modelled to run centre-line to centre-line: they should not

stop at the face of the adjacent wall, nor should they run beyond the centrelines.

Shear Wall parameter in mesh object Consider whether wall should be considered a “shear wall” or not (i.e. Does it take

horizontal force from the slab?) because a shear wall in RAM will take some of the

post-tension force. A core wall should be considered a shear wall; a wall with a

sliding connection is not a shear wall.

Slab continuity through a wall Consider whether the slab is really continuous over the wall.

For example, in the case of an isolated stair landing or lift lobby, most likely it will

not be continuous, as most buildings are constructed with jump (or slip) forms and the

slabs on either side of the walls are connected to the walls with couplers or

rebox/kwikastrip.

If the slab is not continuous, then model two separate walls or line supports, one

supporting the slab on each side, with a void in between; or else leave the smaller slab

out completely and design it by hand.

Theoretically, bars cast into walls can take some moment but they are rarely

constructed correctly, and so care should be taken when deciding whether to make use

of such moment capacity in the model.


Walls above which begin at the slab

Consider whether they are loading the slab or supporting it

If the wall is supported by the slab, do not include in model as they will incorrectly

take moment out of the slab. Apply line loads on the slab for the dead and live load

being applied by the wall.

In some cases (e.g. an outstand wall connected to the lift core) the wall may actually

be a hanging support, in which case it can be modelled as a wall under or with a line

support. Note that careful detailing is required for the reinforcement connecting the

slab to a hanging support.

If designing a footing or core cap in RAM, do not model the walls over as they will

cause errors and unconservatively-low slab moments.

For walls above used to limit deflections, see SERVICE DESIGN – Long Term

Deflections.

Concrete Balustrades (and similar thin upstands)

Needed for slab to work?

NO – do not model but apply line loads on the slab for the dead and live loads applied

by the upstand

YES – model as a beam object and have RAM calculate reo for the upstand beam

Note: for such a design to be valid, the edge of the slab must be fully propped

until the upstand beam gains strength.

Slab Folds

Is slab continuous through fold?

YES – include fold in model, run design strip along fold to check whether it needs

heavier-than-typical longitudinal reinforcement (the fold will attract moment

according to its stiffness in relation to the slab), detail the same reinforcement

required in the slab to run properly-developed across the fold – if the fold is thinner

than the slab, check that it has enough capacity to carry the slab moment through

NO – model the portions of slab on either side of the fold separately to span simply-

supported onto the fold, design the fold by hand as a beam supporting the slabs

Pour Strips, Construction and Movement Joints

Effect of pour strip: Main slab area will be poured and then post-tensioned; all of the

shrinkage due to the post-tensioning will have occurred before the pour-strip is filled

in, and thus the PT force is not transferred through the pour strip. Model the pour strip

concrete as a slab with very low axial stiffness in the direction of the PT (Set the

appropriate stiffness factor to 0.001 under the “Behaviour” tab in the slab properties).

Effect of construction joint: Slab is continuous but no PT force can be transferred

(unless couplers are used); Model using the same method as for a pour strip, stopping

the tendons either side of the C.J.


Effect of movement joint: Shear is transferred but no moment, structure works by

shorter span cantilevering to pick up longer span; Model using a strip of slab with

very low axial and bending stiffness in the direction of the span. Note: the shear force

which the movement joint is designed to take (i.e. by means of a corbel or shear

connectors) should be calculated by hand based on an appropriate tributary width of

the slab that spans onto the movement joint.

Temporary penetrations (such as for cranes and hoists)

Model as a void with the future infill loading applied as a line load around the edges

(the only case in which it would be correct to model the void in its future filled-in

state is if the surrounding slab were propped until the void was filled in)

LOAD COMBINATIONS

Service live load factors Live loads can be modelled on the Live Load (reducible) plan if a short term live load

factor of less than 1 is appropriate (this is the only difference between reducible and

unreducible plans).

The Max Service load combination should have a factor of 1 for live loading as the

creep factor we calculate and use in “Calc Options” will already be adjusted for the

transitory nature of the live load if appropriate (see CREEP FACTOR).

Wind/Seismic loading If possible, specify these as separate loadings and select one of RAM’s default

loading types (such as Ultimate Seismic or Service Wind) – when these are used,

RAM will automatically generate appropriate load combinations, and the load factors

may be adjusted if needed.

LOADINGS

Area Loads Draw area loads neatly so that the model can fit on one page (some model inputs will

be included in design reports), but don’t have to exactly follow the slab outline.

Horizontal Loads While it is sometimes preferable to calculate the stresses and reinforcement due to

horizontal loads separately, they can still be applied to the model in order to note any

unexpected effects they may have. If horizontal loads are applied they should be

applied at the centre of the slab or else they will induce secondary moments.

PATTERN LOADING

Pattern loading must be considered if there are cantilevers or high live loads, or if the

design fits the criteria set out by the relevant design code. However even if this is not

the case, it is still advisable to consider pattern loading as it will help the designer to

ensure that the most adverse situations are covered by the design.

Go to the “Pattern” plans and draw pattern loads over the areas that are considered

loaded for each pattern. The On and Off-pattern factors in “Loadings” (normally 1

and 0 respectively) determine how much of the load is considered in the on and off

patterned areas.


CREEP FACTOR

RAM Concept is able to calculate long term deflections which take into account creep

and shrinkage using a user-entered design creep factor and shrinkage strain. The

values entered for these parameters, particularly the creep factor, have a significant

effect on the calculated deflections, especially if the slab has a low degree of cracking.

Note that the Creep Factor does not affect the calculation of cracking.

The values for the Creep Factor and Shrinkage Strain should be entered in the

appropriate boxes on the window brought up by clicking “Calc Options…” under the

“Criteria” menu.

Whether you are creating a new model or you have started to use or check an

existing model, the Creep Factor should always be considered.

Design creep factor The default values of 3.35 for the Creep Factor and 0.004 for the Shrinkage Strain are

based on the American code. Appropriate values for Australian designs can be

calculated using AS3600 clause 3.1.8, where the Creep Factor to be entered into RAM

will be φcc + 1 (adjusted for transitory nature of live load, see below), and clause

3.1.7, where the Shrinkage Strain to be entered into RAM will be εcs. Appropriate

values for British code designs can be found in clauses 7.3 and 7.4 of BS8110-2,

where the Creep Factor is equal to 1 + Φ (the creep coefficient) from Figure 7.1 and

the Shrinkage Strain is equal to the drying shrinkage from Figure 7.2

Note that the code creep factors are increased by 1 before entering them into RAM.

This is because the Creep Factor is used to factor up the short term deflections

calculated by RAM which take cracking into account. Therefore the additional 1

represents the short term component of the total deflections.

Transitory live loads Concrete creeps under loadings that are sustained over a period of time. Permanent

loads like self-weight will be in place for the entire life of the structure, however live

loads are generally not permanent, unless they are for things like storage or plant

equipment. It would thus be overly-conservative to apply the full design creep factor

For Australian designs, the RBG Creep & Shrinkage Spreadsheet can be used to

quickly calculate the Shrinkage Strain and the Creep Factor and includes formulae to

adjust for live load transience. For British designs, these parameters are taken directly

from graphs in the code; the Creep Factor can be adjusted for live load transience

using the following method:

1. ΦLL = ψl * ΦDL + (ψs – ψl)

2. % live load = q / (g + q)

3. Φ for RAM = % live load * ΦLL + [(1 - % live load) * ΦDL]

where ψs = 0.75 and ψl = 0.25 according to BS8110-2:1985 Clause 3.3.3


DESIGN STRIPS

What is a design strip?

A design strip allows the user to quickly apply several (usually around 10) design

section cuts to the slab along a desired span. Analysis and design calculations are

carried out at each of these section cuts. The design actions used are the averages of

the values across the design section, so the distribution of moment and shear across

a section cut is extremely important.

Tip on placement of design strips:

Using the “Plot” button on the toolbar, display the ultimate moment contour plot in

the background as you place design strips (Note: for bending left to right across the

screen, select “Y axis” and for bending up and down the screen select “X axis”). This

will help you to visualise the slab behaviour and should result in better design strip

widths and positions and therefore better results.

Auto-generate function DO NOT USE! Designer should manually place design strips – this shows

understanding of the structural system being used, and ensures no design strips are

ignored. Setting up design strips should be the longest step in the design process

when using RAM Concept as incorrect design strips can invalidate your results!

Location Design strips should be placed so that all critical design locations are checked.

In a PT slab they should generally be placed in order to cover the entire slab, so that

every tendon is included in the calculations and no areas with either too little or too

much P/A are overlooked. However if a slab is one-way with constant-profile tendons

in the secondary direction for crack control, design strips are probably not required in

the secondary direction as long as the designer checks that the distribution of P/A is

adequate (see POST TENSIONING).

For an RC slab it may not be necessary to place design strips everywhere, particularly

if there are less critical areas where the reinforcement will be rationalised or if the

floor and its loading arrangement are symmetrical. The advantage of placing fewer

strips is that the program will run faster because it is making fewer calculations. This

should be weighed against the fact that the program will refine the finite element

mesh everywhere that there are design strips which will produce more consistently

sized finite element and more nodes.

(In addition, the newest version of RAM uses Load History calculations to produce

deflection contour plots that take into account creep, shrinkage, cracking, and tension-

stiffening; this requires as much of the slab as possible to be covered with design

strips; this is not as yet covered by this guide)

Strip Widths The determination of column and middle strip widths should begin with the maximum

allowed by the design code. The designer should then examine each strip to ensure

that the widths used will result in appropriate design action averaging across the width

of each design section. In some cases it may be necessary to reduce the width of a


strip for this reason It is best to use manually-entered span and strip boundaries

(displayed by purple and yellow lines respectively).

In some cases compressive problems in PT slabs may be alleviated by widening the

design strip. This situation most often arises where span lengths differ significantly.

Theoretically, the shorter span should have a narrower column strip, however it may

be permissible to use the same column strip width for all the spans. Engineering

judgement should be used in all such situations.

Lengths Design strips should usually be drawn between the centrelines of supports. If the

support grid is irregular then the design strips should still be drawn straight left and

right or up and down, so that the design sections generated by RAM are orthogonal to

the reinforcement and tendons being placed. In such cases the strip may (correctly)

end at a line of support rather than an actual support.

If the strip ends at an actual support, RAM will by default take the first design section

at the face of the support by means of the “Support Width” design strip property. If

the support width is set to zero, the first section will be taken at the end of the design

strip, if a value greater than zero is entered then it will be taken at half that distance

from the end of the strip.

It is an important matter of engineering judgement as to whether to take the first

design section (and therefore the critical negative moment and shear) at the face

of the support or at some distance into the support.

AS3600 clauses 6.1.4 and 6.2.3 indicate that in fact the critical section should be

within the column (the corresponding clause in BS8110-1 is clause 3.7.2.6), however

this may not be an issue for smaller columns. In some cases (such as long blade

columns) the negative moment can be close to zero at the column face (remember, a

column is actually a point support in RAM), which would clearly require the critical

section to be located further into the column.

Cover to reinforcement Consideration must be given to the laying sequence of the reinforcement when design

strips are being drawn. The concrete cover property in the design strips cannot be the

same in both directions.

Slab thickness changes Design sections with different slab thicknesses across their width will lose the thinner

slab area for shear calculations. It may be necessary to change the boundary of the

design strip, or even to use separate design strips if unrealistic amounts of shear reo or

shear failure errors are being generated.

Design strips with different slab thicknesses along their lengths are acceptable as long

as an appropriate “Inter Cross Section Slope Limit” is used (usually 0.25 – found

under column or middle strip tab, in design strip properties). This property determines

how rapidly the design sections become deeper or shallower as the slab does so, and is

used because a thicker concrete element such as a drop panel does not become

immediately effective at the point where the slab thickens – this effectiveness

increases gradually as the stress “flows” into the thicker section. A slope limit of zero


will result in all design sections in the deeper concrete being the same depth as those

in the shallower concrete.

Supported ends Strips must be considered supported at each end, unless it is a cantilever or it is

continued with a separate strip. It is often necessary to manually select this in the

design strip properties, especially with an irregular column grid where a design strip

spans onto a slab strip running in the other direction rather than onto a column.

Two-way flat slabs RAM does not address for the requirement of AS3600 clause 9.1.2 where 25% of the

design strip negative moment must be carried by a section of width D either side of

the column. When designing two-way slabs, the designer can manually meet this

requirement by adding up the column and middle strip moments as displayed on a

strip plot, and then enter them into the appropriate RBG spreadsheet available on the

intranet (called RBP-SS-AS-3600 9.1.2+9.2).

REINFORCEMENT

Viewing required reinforcement A strip plot of the area of steel in mm

2 for each section cut can be viewed under

section design – viewing this plot on the same plan as the actual bar call-ups can help

the designer to determine exactly where the reo is needed, and can help to

differentiate between reo that is actually needed, and errors such as reo being called

up at the free edge of a slab.

User-defined reo

Can be specified as a % in the design strip or applied as actual bars in the

Reinforcement plans (bars default as orange-coloured)

Advantages of using Reinforcement plans instead of % reo in design strips:

– quicker and more user-friendly

– more control over placement of reo (specifying top reo in a design strip

applies that top reo across the entire strip whereas we will often only want it

near a support)

– can specify anchorage type

– can specify expected reo or mat of reo and then clearly see where any

additional reo is required by showing it on the Reinforcement plan

AS3600 Clause 8.1.4.1 – min reo

RAM carries out the 8.1.4.1 calculation at sections near supports and at the mid-span

but doesn’t distinguish between column and middle strips; such reo may be ignored

by the designer if the section is not considered critical.

AS3600 Clause 8.1.3 – ductility

This clause limits the depth of the neutral axis to ensure a ductile failure of the

section, and RAM tries to meet this requirement by calling up compression reo. This

generally occurs in PT slabs where the section is thin or the concrete strength is low.

Reo and failures called up under 8.1.3 should not be ignored.


Any reo called up under 8.1.3 must be provided, or else the design altered. If the

section fails under 8.1.3 then the design must be altered.

Note: predicted deflections which are somewhat greater than the desired limit are

more preferable than a brittle failure of the structure, especially when you consider

that long term deflections are based on a creep factor which can be off by +/- 30%!

POST TENSIONING

The following points should be considered when modelling a post-tensioned slab:

• Consider layering of tendons and reo, clashes between tendons, and max and

min allowable profiles (tendons in both directions should not be at the same

height and should be far enough from the surface/soffit to allow any reo to be

laid)

• Consider anchorage locations (obtaining information about the construction

sequence and methodology will help you to locate anchors correctly)

• Jacks must be provided on at least one end of each tendon – without these

jacks, tendon losses will not be modelled

• The balance load plans should be inspected to ensure that the appropriate P/A

is being distributed throughout the slab

• RAM enables the designer to calculate the PT rate using “Estimate” (found

under Report menu) – simply take the kg of PT and divide by area of the

formwork, this gives kg/m2 of PT (cannot be done for reo). Note that the area

of formwork excludes penetrations so the calculate rate will most likely be

higher than the rates specified in our preliminary designs

• RAM calculates PT strand extensions – using this tool avoids the need for

separate spreadsheet calculations (use the Eye to view “jack elongation” on

tendon plan)

Further to the above, the following points should be considered when drafting will be

carried out by tracing over a tendon layout exported directly from RAM:

• Follow profiling convention so the drafters can get it right first time – the

convention should be confirmed with the drafters prior to the first mark-up

being drafted! (Difference between RAM profiles and actual profiles are: High

Point +5mm, Low Point +15mm, Slab mid height +10mm, Anchorage no

difference – These are for flat (slab) ducts; multi-strand ducts will have very

different profile differences).

• Place tendons, profile points, and anchorages accurately throughout the design

process so they don’t have to be fixed up after the drawing is drafted –

Drafters shouldn’t be expected to know where they should be!

• Check that you haven’t left any tendons with incorrect strands (e.g. you may

have experimented quickly by putting in 10-strand tendons to simulate

multiple tendons, but when it goes for drafting obviously 2 separate 5-

stranders need to be shown).


SERVICE DESIGN

Viewing Deflections

Contour plots RAM does not consider creep or cracking for its contour plots (unless Load History

calculations are carried out using the latest version of RAM – these are not covered in

this guide), so these deflections are only elastic (short-term). A long-term deflection

contour plot is based on a special load combination, with load factors increased to

allow for creep and cracking. This method is not as accurate as strip plots, but can be

used to generate graphical presentations for clients.

Strip Plots If “detailed section analysis” is ticked under the service design rules, RAM will

perform a cracked section analysis at each design section. The calculation takes

account of the effect of creep and cracking based on the creep factor entered in “Calc

Options”, and plots long term deflections along each design strip (as well as short

term deflection, degree of cracking (ECR), and top and bottom crack widths).

Serviceability considerations

Long-Term Deflections

Spanning between columns

Due to columns as actually being point supports in the finite element model, slab

deflections can usually be seen to begin “within” the column. If slab deflections are

too great with the columns just modelled as pins and more accurate deflection

calculations are required, this can be achieved using the following steps:

1. Model a wall above the column – this will stiffen the slab at the column and

reduce deflections but will correspondingly attract more negative moment –

reinforcement will then be designed for this increased moment.

2. If step 1 does not improve deflections enough, then assign bending stiffness

to the column below (keeping the wall above in place) – this may cause the

column to take moment from the slab which will further improve deflections

but will require a check that the column can take the calculated moment.

Spanning between beams

Maximum deflections should be adjusted to account for the deflection of the beams,

which can sometimes cause the actual differential deflection in the slab to be less than

the maximum span deflection calculated by RAM.

Note on deflections that differ in the same location:

If deflections differ where two design strips in different directions meet (such as

where a slab spans onto a slab strip in the orthogonal direction), this should be

investigated. This phenomenon is the result of different degrees of cracking in each

direction. It will be conservative for the supporting slab strip to take the greater

magnitude deflection as governing. However the mid-span deflection of the slab strip

which is spanning onto it may actually be higher than the value displayed. See Section

58.8 pages 521-525 of the RAM User Manual)


Incremental Deflections

It will be sufficient to take the incremental deflection as being the difference between

the total long term deflection and the elastic deflection.

Note that if cracking occurs due to construction loading, the long term deflections

predicted for the service case may be unconservative.

Cracking and Crack Widths

AS3600 – crack widths do not need to be considered. If deflections are within limits

and the reinforcement required under cl’s 9.4.1/2 is provided (based on amount of

tensile stress in the concrete), cracking is considered to be controlled.

It should be noted that the predicted crack widths may be noticeably higher than the

BS8110 limits, and yet the deflection and cracking reo requirements are being met. If

the visibility of cracks is critical (for example a tiled floor or rendered ceiling) then

the designer may still wish to consider crack widths, and endeavour to reduce them by

adding reo or altering the PT.

Note: Reo calculated by RAM has already been taken into account for the calculation

of crack widths, so reo greater than that calculated by RAM must be specified by the

user if it is to be used to improve the crack width.

BS8110 – crack widths are to be limited to 0.3mm for RC slabs and 0.2mm for PT.

RAM will calculate reinforcement designed to achieve the required crack width (and

will also check the recommendations of TR-43 for PT slabs). Again, additional user

reinforcement can be added to improve the crack widths if desired.

Effect of reinforcement on service performance The degree of cracking in a slab has a great influence on deflections. Therefore

adding user reinforcement to reduce the degree of cracking will usually improve the

deflection.

A convenient way to gauge the degree of cracking is viewing a plot of the effective

curvature ratio, or ECR (found under section analysis, like long-term deflection). On

one plot, the degree of cracking at both the supports and the midspan can be viewed –

ECR values higher than the creep factor indicate a cracked section.

Note: the locations of PT tendon high points and low points will also have a great

impact on cracking and deflections, and incorrect tendon profiles can often be the

cause of cracking/deflection problems.


ULTIMATE DESIGN

Punching Shear

• For Australian design, use moments from the support reactions. These should

be calculated using a separate model with column stiffnesses = 100%. Note

that AS3600 has a minimum M*v requirement (see clause 6.10.4.5) and

punching shear calculations should be carried out using whichever moments

are worse, those from the code or those calculated by RAM. Zero moment at a

column should never be assumed when carrying out punching shear design to

the Australian code.

• To determine σcp use the average pre-stress across the shear perimeter which

can be viewed separately for each direction on the Balance Loading plans.

• For British design, moments are not used but the shear force is factored up

depending on column location, so only one model is required.

TROUBLESHOOTING

Model takes too long to run Model may be too large – Break it into smaller separate models, but include in each

model 2 spans beyond the boundaries to correctly model continuity. Ignoring these

additional spans and simply using a line support with fixed rotation does not model

this correctly.

Structure not stable Tick “Auto-stabilize structure in x and y directions” in Calc Options.

Shear Failure Design strip “shear core” too small – check that design strip sections don’t have

multiple slab thicknesses; this is clearly indicated in the cross sections perspective by

dark coloured areas of concrete, which are not used for shear design (they are used for

flexure).

Cracking Failure Design strip section too shallow – check Inter Cross Section Slope Limit (should be

0.25) which allows for the progressive distribution of stress from a thinner section

down into a thicker section.

Ductility Failure See comments on clause 8.1.3 under the heading “Reinforcement”

Flexural failure Can result from a design section not being intersected by a tendon – in ultimate

design, design sections not intersected by tendons ignore the strength effects they

provide (P/A that spreads out from a tendon is only considered for service design)

Deflections of different magnitudes in each direction at the same location This is not an error, but rather is the result of the slab having a greater degree of

cracking in one direction than in the other direction (see SERVICE DESIGN – Long

term deflections) and Section 58.8 of the RAM Concept Manual.


Strip fails: “Exceeded TR43 6.10.1” (BS 8110 design only)

Refers to Clause 5.8.1 in the latest (2nd

) edition of TR43 - similar to AS3600 cl

9.4.1/2; Simply check that the crack widths as calculated by RAM are acceptable

(generally limited to 0.3mm for RC slabs and 0.2mm for PT slabs, see BS8110-2 cl

3.2.4 and BS8110-1 cl 2.2.3.4.2).

Note that RAM has used the calculated reo to determine those crack widths, so if less

reo is being put in then the crack widths will not be correct; see “Service Design”

section above.

Load combination factor does not look correct for strength design etc Intended to prevent “mistakes” where designer has manually specified load

combinations, such as for earthquake or wind loads (see “Load Combinations” section

above).

If the designer has carefully considered any unusual and/or manually-entered load

combinations, and the load factors and design rules being used are correct, then this

error can be ignored.

Other error messages The RAM Concept User Manual explains each of the error messages that the program

generates – see Section 38.

rbg ram concept guide

Documents

guide version

latest version of ram

rbg guide

design strips

ram concept page

concrete balustrades

slab folds

service design